Test specimen and device for characterizing the vibration damping of a contact between two parts

Abstract

Test specimen and device for characterizing vibration damping of a contact between two parts. One aspect of the invention relates to a test specimen (100) for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibratory, oligocyclic and tribological stresses, the specimen comprising a male-female type attachment (130) including a male part (131) projecting from the free end of a first longitudinal bar and a female part (132) notched at the free end of a second longitudinal bar.A second aspect of the invention relates to a device (300) for characterizing the vibration damping of an adherent or sliding contact between two parts subjected to low-cycle, tribological, and vibratory stresses. The device comprises a test bench (300) on which the test specimen (100) is mounted and includes a vibrating pot (310) that transmits vibrational excitations to the test specimen (100) and a static loading device (320) that transmits low-cycle stresses to said test specimen. A third aspect of the invention relates to a method (400) for implementing the characterization device (300). Figure to be published with the abbreviation: Figure 7.

Description

Title of the invention: Test specimen and device for characterizing the vibration damping of a contact between two parts. TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a test specimen representative of an adherent or sliding contact between two parts, such as a blade root and a cell of a turbomachine turbine disk. It also relates to a device for characterizing the vibration damping of the adherent or sliding contact between these two parts, using this test specimen. The invention further relates to a method for implementing this characterization device.

[0002] The invention finds applications in the field of characterizing the connection between blade roots and the cells of turbomachine disks and, in particular, in the field of characterizing vibration damping of the adherent or sliding contact between a blade root and a turbine disk cell. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] In aeronautics, it is known that a turbomachine comprises a combustion chamber and a set of turbines arranged downstream of the combustion chamber to recover the energy from the combustion of the gases. A turbine, in particular a low-pressure turbine (or LP turbine) or a compressor, a fan (FAN), or a high-pressure turbine, comprises a rotor formed of a set of disks and blades assembled around a central shaft. An example of a portion of a disk 20 and some movable blades 10 is shown in [Fig. 1]. The movable blades 10 are arranged radially around the disk 20, as shown in [Fig. 1], the disk 20 itself being mounted around the central axis extending along the X direction of the orthonormal XYZ coordinate system.

[0004] A movable blade 10 comprises a blade 11 extending radially, substantially along the Y-axis, between a proximal end 12 (or inner end) near the disk 20 and a distal end (or outer end), not visible in the figure and terminating in a heel. At the proximal end 12 of the blade 11, the blade 10 includes a foot 13 for attaching said blade to the rotor disk 20. For this purpose, the disk 20 has recesses 22 distributed radially around its circumference, each receiving the foot 13 of a blade 10. Each recess 22 has a shape complementary to that of the blade foot 13. In the example of [Fig. 1], the feet 13 of the blades 10 have a so-called "fir tree foot" shape with saw-toothed threads. and the 22 alveoli of the 20 discs have a sawtooth shape, complementary to the shape of the blade feet.

[0005] When the movable blades 10 are mounted in a ring on the disk 20 and the turbine is running, the blades 10 are subjected to complex stresses, combining vibrational stresses with low-cycle stresses, leading to damage to the blades. Indeed, many parts of the turbomachine are subjected to more or less complex stresses. In particular, as shown in [Fig. 2], each turbine blade 10 is subjected to drag and lift forces as well as a centrifugal force, due to the rotation of the turbine and the origin of the low-cycle stresses on the blades. The turbine blades 10 are also subjected to vibrational excitation due to the environment and / or blade resonance, this vibrational excitation being able to generate deformation of the blades 11 as shown in [Fig. 3], or even a crack in the blades when the vibration level is too high.The centrifugal force combined with thermal and vibratory stresses can lead, on the one hand, to slippage depending on temperature, span angles or centrifugal stress, between the threads of the blade feet 13 and the walls of the disk cells 22 and, on the other hand, to non-linearities in the contact behavior between blade feet and disk cells.

[0006] To study the behavior of turbomachinery components, and in particular high- or low-pressure turbine blades, compressor blades, or fans, aircraft manufacturers generally conduct tests to characterize their fatigue resistance, especially vibration fatigue, to the stresses and strains experienced by these components. Since testing on actual parts is costly (complex tooling, part prices, lead times), tests are, whenever possible, carried out using test benches and specimens representative of the parts being studied. However, characterizing the stresses on parts subjected to various loading is relatively complex. Characterization becomes even more complex when tribological loading is added, that is, loading occurring between two parts in contact (friction, wear, etc.), as is the case for the attachments between blades and discs.

[0007] For the blade-disk connections, i.e., the interfaces or contact zones between blade roots and disk cells, knowledge of the tribological aspect is essential because the contacts between the blade roots and the disk cells generate vibration damping. Studying and understanding this tribological aspect allows for the development of blade and disk models, with the aim of optimizing the design of a turbomachine, particularly to limit its vibrations. The objective of limiting vibrations between turbine blades and disks, in order to reduce stress on the blade-disk interfaces, requires to know the vibration damping between these parts and, consequently, the vibration amplitude.

[0008] Today, due to its complexity, the characterization of vibration damping between turbine blades and disks is performed on actual parts, with all the drawbacks described above. This characterization is determined using strain gauges to measure the impact of any change in the characteristics of the blade-disc interfaces, such as surface finish, surface treatment, etc. However, tests with strain gauges mounted on actual parts are tedious, time-consuming, and expensive. Moreover, the very presence of strain gauges on the blades interferes with the vibration damping measurements by altering the mass, stiffness, and other properties of the blades being tested.

[0009] There is therefore a real need for a technique that makes it possible to do away with these tests on real parts. Summary of the invention

[0010] To address the aforementioned problems of characterizing the damping of the interface between turbine blades and disks, the applicant proposes a test specimen representative of the interface between a blade root and a turbine disk cavity, adapted for characterizing the vibration damping of the adherent or sliding contact between these two parts. The applicant also proposes a characterization device using this specimen, as well as a method for implementing this device. The device and method allow for measuring the intrinsic vibration damping of the interface, without any disturbance or influencing factor external to said interface.

[0011] According to a first aspect, the invention relates to a test specimen for characterizing the vibration damping of an adherent or sliding contact between two parts subjected to vibrational, oligocyclic and tribological stresses, the test specimen comprising: • a first head and a first longitudinal bar, one end of the first longitudinal bar being fixed to the first head, the other end of the first longitudinal bar being free, • a second head and a second longitudinal bar, one end of the second longitudinal bar being fixed to the second head, the other end of the second bar being free, and • a male-female type attachment connecting the first longitudinal bar with the second longitudinal bar and comprising a male part projecting from the free end of the first longitudinal bar and a female part notched at the free end of the second longitudinal bar, the male part and the female part comprising forms complementary to each other, in adherent or sliding contact against each other.

[0012] This test specimen is representative of the interface between a blade root and a turbine disk cavity and can be subjected to the same stresses as those experienced by a real interface. This test specimen can therefore be used to characterize the vibration damping of the interface between blades and turbine disk and obtain a "pure" value free from external disturbances that could distort the measurements.

[0013] In addition to the characteristics mentioned in the preceding paragraph, the test specimen according to one aspect of the invention may have one or more additional characteristics from among the following, considered individually or in all technically possible combinations: • the first longitudinal bar has a shape roughly like a rectangular prism, with a flare near the first head and a shoulder at its free end. • the second longitudinal bar has a shape roughly like a rectangular prism, with a flare near the second head and a buttress at its free end. • the shoulder of the first longitudinal bar and the buttress of the second longitudinal bar extend in a plane perpendicular to a plane containing the flares of the first and second longitudinal bars. • The male-female type attachment has substantially a dovetail shape representative of a connection between a blade foot and a turbomachine disk cell, the male part representing the blade foot and the female part representing the disk cell. • the test specimen is a solid reference specimen with no contact zone.

[0014] A second aspect of the invention relates to a device for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibrational, oligocyclic and tribological stresses, the device comprising a test bench on which the test specimen according to the first aspect is mounted, said test bench comprising at least a vibrating pot transmitting a vibrational excitation to the test specimen and a static loading device transmitting oligocyclic stresses to said test specimen.

[0015] This characterization device makes it possible to eliminate any vibrational disturbance by elements other than the blade / disk interface itself; it therefore allows a measurement of the "pure" value of the intrinsic damping of the contact areas studied, that is to say the vibrational damping resulting from the sliding at the contacts between blade roots and disk cells.

[0016] In addition to the characteristics mentioned in the preceding paragraph, the device according to the second aspect of the invention may have one or more additional characteristics from among the following, considered individually or in all technically possible combinations: • The vibrating pot is connected to the test specimen by means of a threaded excitation rod, in which a force measurement sensor is integrated in an off-center manner. • The excitation rod comprises a first and a second section, joined together by an off-center linking system, the first and second sections having different lengths.

[0017] A third aspect of the invention relates to a method of implementing the device according to the second aspect, comprising the following steps: a. Installation of a reference test specimen in the characterization device, b. Adjustment of the characterization device according to the measurements to be performed, c. measurements of the vibrational behavior of the reference specimen, d. removal of the reference test specimen, e. Installation of the test specimen according to the first aspect in the characterization device, with the same adjustment as in step b), f. measurements of the vibrational behavior of the test specimen, g. comparison between the measurements carried out in step c) for the reference specimen and the measurements carried out in step f) for the test specimen, and h. determination, from the comparison of step g), of the characterization of the vibration damping of the contact zone.

[0018] Advantageously, the method includes a preliminary step of temperature calibration of the reference specimen and the test specimen in a furnace. BRIEF DESCRIPTION OF THE FIGURES

[0019] Other advantages and features of the invention will become apparent from the following description, illustrated by the figures in which:

[0020] Fig. 1, already described, represents a perspective view of a portion of a turbine blade and disk assembly;

[0021] The [Fig.2], already described, represents a partial perspective view of a moving turbine blade;

[0022] Fig. 3, already described, represents side and top views of a moving turbine blade;

[0023] Fig. 4 represents perspective views of a test specimen according to the invention;

[0024] Figure 5 shows perspective views of a reference test specimen used in the process according to the invention;

[0025] Figure 6 shows a schematic view of the test specimen attachment area of ​​the [Fig.4] with the demands placed on it;

[0026] Figure 7 represents the characterization device according to the invention of a point of functional view, showing the stresses applied to the test specimen;

[0027] Figure 8 schematically represents the characterization device according to the invention, in which is mounted the test specimen of the [Fig.4];

[0028] Figure 9 shows a side view of a threaded excitation rod offset along an embodiment of the device for characterizing the invention;

[0029] Figure 10 shows a schematic side view of an excitation rod along a preferred embodiment of the device for characterizing the invention;

[0030] Figure 11 represents, in the form of curves, examples of measurements of the displacement amplitude and natural frequency of a test specimen according to the invention; and

[0031] Figure 12 represents, in the form of a functional diagram, the process according to the invention.

[0032] Unless otherwise specified, identical elements are identified in the figures by identical reference numerals. For the sake of clarity, the size scales between represented elements are not necessarily to scale. DETAILED DESCRIPTION

[0033] An example of the embodiment of a test specimen, allowing the characterization of vibration damping between two parts in adherent or sliding contact, and of a characterization device comprising this specimen, are described in detail below, with reference to the accompanying drawings. This example illustrates the features and advantages of the invention. It should be noted, however, that the invention is not limited to this example.

[0034] An example of a test specimen 100, representative of a contact between two parts 110, 120 such as a turbine blade and a turbine disk, is shown in [Fig. 4]. The two contacting parts 110, 120, represented by the test specimen 110, can, in particular, be a movable blade root 13 10 and a groove 22 of a turbomachine turbine disk 20. As explained previously, the contact between a blade root and a disk groove is a friction or sliding contact that generates vibration damping. The test specimen 100, more simply called the specimen, is designed to represent this friction or sliding contact between a blade root and a disk groove. For this purpose, the specimen 100 comprises two parts: a male part 110 and a female part 120.

[0035] The male part 110, shown in drawing A of [Fig. 4], comprises a head 111, called the first head, and a longitudinal bar 112, called the first bar. The first head 111 is a part of the test specimen 100, relatively massive compared to the first bar 112, and designed to be fixed to a characterization device or a test bench. The first head 111 can take all sorts of shapes, such as a square or rectangular parallelepiped, or a cylindrical shape, etc. The first bar 112, integral with the first head 111, extends from said first head 111 to the female part 120. The first bar 112 has an end 112a integral with the first head 111 and a "free" end 112b, in adherent or sliding contact with the female part 120.The first bar 112 has a shape essentially of a rectangular prism, that is to say, a parallelepiped with a rectangular cross-section, smaller than the cross-section of the first head, and whose fixed end 112a is flared and whose free end 112b is provided with a shoulder 113. The first bar 112 therefore has a flare 114, in the area of ​​connection with the first head 111, which extends laterally (along the lateral direction 1) relative to the longitudinal direction L of the central portion 112c (in the shape of a rectangular prism) of said first bar 112. The shoulder 113 of the first bar 112, arranged near the connection area 130', or interface, with the female part 120, is a portion of the first bar 112 projecting beyond the central portion 112c and extending transversely (along the direction T) with respect to the longitudinal direction L of the central portion 112c. .

[0036] The female part 120, shown in drawing B of [Fig. 4], comprises a head 121, called the second head, and a longitudinal bar 122, called the second bar. The second head 121 is a relatively massive part of the specimen compared to the second bar 122 and is designed to be fixed to a characterization device or a test bench. The second head 121 can take on all sorts of shapes, such as a square or rectangular parallelepiped, or a cylindrical shape, etc. The second bar 122, integral with the second head 121, extends from said second head 121 to the male part 110. The second bar 122 has an end 122a integral with the second head 121 and a "free" end 122b, in adherent or sliding contact with the male part 110.The second bar 122 has a shape essentially of a rectangular prism, that is to say a parallelepiped with a rectangular cross-section, smaller than the cross-section of the second head, and whose fixed end 122a is flared and the free end 122b is provided with a buttress 123. The second bar 122 therefore has a flare 124, in the area of ​​connection with the second head 121, which extends laterally (along the lateral direction 1) with respect to the longitudinal direction L of the central portion 122c (in the shape of a rectangular prism) of said second bar 122. The buttress 123 of the second bar 122, arranged in the vicinity of the area of ​​connection. 130' with the male part 110, is a projecting reinforcement, flaring out along the transverse direction T, relative to the longitudinal direction L of the central portion 122c.

[0037] The male part 110, like the female part 120, is a single piece, machined from a single unit. The male part 110 and the female part 120 may be partially identical, with only the portions of the first and second bars 112, 122 near the free ends 112b, 122b being different. One of the differences is the shoulder 113 for the male part 110 and the buttress 123 for the female part 120. Another difference may be the difference in length between the first bar 112 and the second bar 122; in the example illustrated [Fig. 4], the central portion 122c of the second bar is longer longitudinally than the central portion 112c of the first bar. However, the geometries of the male part 110 and the female part 120 may vary depending on the desired deformations. Thus, the two bars 112 and 122 can be of the same length, or the second bar 122 can be shorter than the first bar 112.Their width or thickness can also be the same or different.

[0038] As shown in drawing C of [Fig. 4], the male part 110 and the female part 120 are joined together by means of a male-female connector 130. This male-female connector 130, more simply called the connector, provides a friction or sliding contact between the first bar 112 of the male part and the second bar 122 of the female part. The connector 130 forms the interface 130' between the male part 110 and the female part 120 of the test piece 100. For this purpose, the connector 130 comprises a male portion 131 positioned on the male part 110 and a female portion 132 positioned on the female part 120. The male portion 131 and the female portion 132 are substantially dovetail-shaped. For this purpose, the male part 131 forms a flared tenon, mounted at the free end of the first bar 112, on the distal wall 113d of the shoulder 113, that is to say the wall of the shoulder furthest from the first head 111.The tenon of the male part 131 has a rectangular parallelepiped shape, one end of which is fitted against the shoulder 113 and the other end is flared in the transverse direction T. The female part 132 forms a notch, cut at the free end of the second bar 122, on the distal wall 123d of the buttress 123, that is to say the wall of the buttress furthest from the second head 121. The notch of the female part 132 has a shape complementary to that of the tenon of the male part 131. Once assembled, the male part 131 and the female part 132 of the fastener 130 form the adherent or sliding contact of the specimen 100.

[0039] The test specimen 100 described above can be used in a test bench or characterization device such as the one described below. This test specimen 100 is used to characterize the vibration damping of the contact adherent or sliding contact formed at the interface 130' of the male part 110 and the female part 120 by the fastener 130. For this purpose, the measurements carried out on the test specimen 100 can be compared to measurements carried out on a reference specimen 200. This reference specimen 200, shown in [Fig. 5], is essentially identical to the test specimen 100, except that it is monobloc, that is to say, manufactured in one piece, without adherent or sliding contact at the interface between the male part 210 and the female part 220. In other words, the reference specimen 200 has the same characteristics of shape, dimensions, and material as the test specimen 100, but the reference specimen 200 has a male part 210 and a female part 220 connected to each other by a fixed interface 230, the shoulder 213 of the male part 210 being fixedly secured with the buttress 223 of the female part 220.The thickness of the fixed interface 230 between the shoulder 213 and the buttress 223 was adapted so that the mass distributions of the male and female specimens are as close as possible.

[0040] As explained above, the test specimen 100 of [Fig. 4] behaves, from a vibrational point of view, in the same way as the assembly between a movable blade root and a turbine disk cavity. It is therefore possible to study the vibrational behavior of this assembly using the test specimen 100 of the invention. In particular, it is possible to characterize the vibrational damping of the adherent or sliding contact of this assembly using the test specimen 100. Indeed, the interface 130' between the male part 110 and the female part 120 can be subjected to the same stresses as the assembly between a movable blade root and a disk cavity, these stresses or forces being applied to the test specimen 100 by means of a characterization device described below. As shown in [Fig.6], the test specimen 100 can be subjected, at the interface 130', to the following forces: • a tribological effort broken down into: • a sliding force, represented by the double arrow G, between the male part 131 and the female part 132 of the interface 130', and • a normal force, represented by the arrow N, perpendicular to the sliding force G, • an oligocyclic stress, or tensile fatigue stress, represented by the LCF arrow and which corresponds to the centrifugal force to which the blades are subjected, and • a high-frequency transverse vibratory effort, represented by the double arrow HCF.

[0041] The characterization device used to apply stresses to the test specimen 100 comprises a test bench 300, an example of which is represented schematically in [Fig. 7], and a calculation unit, not shown in the figures. The test bench 300 can be made by modifying a conventional test bench, such as a fretting-fatigue test bench usually used to determine the life to failure of the attachment between the moving blade and the disk. The test bench 300 of [Fig. 7] is therefore adapted to allow the measurement of the intrinsic vibration damping at the interface between the male part 110 and the female part 120 of the specimen 100, representative of the contact between the blade root and the disk cavity. For this purpose, the test bench 300 includes: • a vibrating pot 310 applying, on the test specimen 100, a vibratory stress HCF along the X axis of the XYZ frame; • a mooring line 333 applying, on the test specimen 100, a normal force N along the Y axis; • an excitation device 320 transmitting, to the test specimen 100, LCF oligocyclic stresses along the Y axis.

[0042] The test bench 300 does not directly apply a sliding force to the specimen 100; the sliding force is obtained by combining the vibrational stress HCF involving contact force components in the N and G directions and the tensile stress LCF which also involves contact force components in the N and G directions. The specimen 100 is thus subjected to all the forces and stresses experienced by the contact between the blade root and the disc cavity.

[0043] A representation of the test bench 300 with the test specimen 100, when said specimen 100 is subjected to the various stresses described above, is shown in [Fig. 8]. This [Fig. 8] shows an upper I-beam 330 comprising the vibrator 310 and the mooring line 333 and a lower I-beam 330' comprising the excitation device 320 and an excitation rod 333'. The specimen 100 described above is arranged between the two upper I-beams 330 and lower I-beams 330'. In [Fig. 8], the specimen 100 is shown when the test bench 300 is in operation, said specimen 100 being subjected to both a vibrational stress HCF and an oligocyclic stress LCF. These two stresses induce contact components in the normal N and tangential G directions to the (planes of) contact.

[0044] In the test bench 300 of the invention, the vibrating pot 310 is coupled to the mooring line 333 of the test specimen 100, which allows a vibration study in a frequency range between 100 and 2000 Hz.

[0045] In conventional test benches, such as those dedicated to fretting-fatigue testing, the excitation rod of the lower I-beam is generally a threaded rod formed in one piece. In the test bench 300 of the device of the invention, the excitation rod 333' is a rod specifically designed for damping characterization. vibratory. As shown in [Fig. 9], this excitation rod 333' is a threaded rod, that is, having a screw thread over its entire external surface, comprising a first rod section 331 and a second rod section 332 joined together by a connecting system 335, for example, of the screw / nut type. The connecting system 335 may, for example, include a threaded tenon 335a, forming a kind of screw at the end of the first rod section 331, and a threaded orifice 335b, forming a kind of nut at the end of the second rod section 332, the threaded tenon 335a fitting into the threaded orifice 335b to maintain the two sections 331 and 332 in contact.

[0046] In certain embodiments, the excitation rod 333' incorporates a force sensor 334, for example a piezoelectric sensor, as shown in [Fig. 10], this force sensor enabling a measurement of the excitation force. In these embodiments, the force sensor 334 is housed in the center of the connecting system 335, between the first and second rod sections 331, 332. The connecting system 335 may then include, for example, a threaded tenon at the end of each of the rod sections 331, 332, each threaded tenon being fixed in a nut integral with the force sensor 334. Of course, other means of fixing the first and second rod sections, with or without the force sensor, can be considered.

[0047] According to certain embodiments, the first and second rod sections 331, 332 are of different lengths, which makes it possible to offset the additional mass represented by the force sensor. Indeed, adding a force sensor 334 to a conventional threaded rod introduces an additional mass that influences the vibratory behavior of the threaded rod, this vibratory behavior itself leading to a modification of the natural mode of the overall excitation system of the test bench. Therefore, offsetting the additional mass generated by the force sensor, as proposed in this embodiment, makes it possible to modify the natural frequencies of the excitation rod 333', the natural frequency thus depending on the difference in length between the first rod section 331 and the second rod section 332.

[0048] According to certain embodiments, the diameter of the excitation rod 333' is larger than the diameter of the threaded rods of conventional test benches. The diameter of the excitation rod 333' of the test bench 300 can, for example, be from 5 to 50 mm. Changing the diameter of the excitation rod 333' allows the stiffness of said excitation rod to be changed, which makes it possible to shift the rod's natural mode outside the measurement range.

[0049] Examples of measurements carried out with the test bench 300 are shown, in the form of curves, in [Fig. 11]. The curves of graph A in [Fig. 11] show examples of measured displacement amplitude and natural frequency. on test specimen 100, in the case of low frequencies (approximately 300-350 Hz), with different excitation rods. The curves in graph B of [Fig. 11] show examples of displacement amplitude and natural frequency measured on test specimen 100, in the case of high frequencies (1800 to 1900 Hz), with different excitation rods. In both frequency ranges (low frequencies and high frequencies): • The Cl curve corresponds to measurements taken with a 300 test bench equipped with a conventional excitation rod (rod Tl shown in part C of [Fig. 11]), • Curve C2 corresponds to measurements taken with a test bench 300 equipped with a centered excitation rod, i.e. formed of two identical sections (rod T2 shown in part C of [Fig. 11]), • Curve C3 corresponds to measurements taken with a test bench 300 equipped with an off-center excitation rod, i.e., formed of two sections of different lengths (rod T3 shown in part C of [Fig. 11]), and • Curve C4 corresponds to measurements made with a 300 test bench equipped with an off-center excitation rod and equipped with a force sensor (rod T4 shown in part C of [Fig. 11]), this excitation rod T4 corresponding to the preferred embodiment of the rod.

[0050] Curve C1 represents the reference curve, i.e., the displacement amplitude measurements in mm and the natural frequency of specimen 100, when specimen 100 is subjected to stress by test bench 300 equipped with a conventional excitation rod TL. Curve C2 shows the displacement amplitude measurements in mm and the natural frequency of specimen 100, when said specimen 100 is subjected to stress by test tank 300 equipped with the centered excitation rod T2. Curve C2 shows in particular that, for high frequencies, the natural frequency of specimen 100 is advantageously displaced, but that, for low frequencies, there is no change in the natural frequency compared to the reference curve. Curve C3 shows the displacement amplitude measurements in mm and the natural frequency of specimen 100, when said specimen 100 is stressed by the test bench 300 equipped with the off-center excitation rod T3.Curve C4 shows the displacement amplitude measurements in mm and the natural frequency of specimen 100 when said specimen 100 is subjected to stress by the test bench 300 equipped with the excitation rod of the preferred embodiment T4. Curve C4 shows in particular that the rod T4 allows, at both low and high frequencies, a very clear shift in the natural frequency of specimen 100. By shifting the natural frequency of specimen 100 relative to the reference natural frequency, the vibrational disturbances of the excitation rod are eliminated in the measurement of the intrinsic damping at . the interface 130' between the male part 110 and the female part 120, regardless of the vibration mode chosen (high or low frequencies).

[0051] Thus, by shifting the natural frequency of the specimen 100, it is possible to move the resonance mode of said specimen away from the resonance modes of the other elements of the test bench such as the excitation rod, the machine structure, the force sensor, etc., in order to ensure that only the intrinsic effect of the slips and contact nonlinearities on the vibratory behavior at the interface of the specimen is studied.

[0052] The characterization device described above can be implemented by a process 400 comprising the following steps: • Step 410: Installation of a reference specimen 200 in the test bench 300, • Step 420: Adjusting test bench 300 according to the measurements to be taken, • Step 430: Measurement of the vibrational behavior of the reference specimen 200, • Step 440: Removal of the reference specimen 200, • Step 450: Installation of test specimen 100 in place of the test specimen reference specimen 200, in test bench 300, maintaining the same settings as those chosen for reference specimen 200, • Step 460: Measurement of the vibrational behavior of test specimen 100, • Step 470: comparison between the measurements taken in step 430 for the reference specimen 200 and the measurements taken in step 460 for the test specimen 100, and • Step 480: determination, from the comparison of step 470, of the characterization of the vibration damping of the interface between the male part 110 and the female part 120 of the test specimen 100.

[0053] In step 420, various adjustments can be made on the test bench 300. Indeed, the test bench of the invention allows adjustment, on the one hand, of the natural frequencies of the test bench / specimen assembly by the choice and dimensioning of the excitation rod 333' and, on the other hand, of the direction of excitation so as to achieve complex resonance modes, which can combine several forces such as, for example, bending and torsion.

[0054] The method 400 makes it possible to resolve a major difficulty in the characterization of vibration damping, which is to isolate the damping of the vibratory mode of the specimen from the damping of the other modes of the device (undesired and inherent to the vibratory behavior of the device), this condition being necessary to measure the damping due solely to contact at the interface 130' between the The test specimen 100 is defined by its male component 110 and its female component 120. In process 400, the vibration mode isolation of the test specimen 100 is achieved by comparing the test results measured with the reference specimen 200 with the test results measured with the test specimen 100. The reference specimen 200 allows for a measurement of the vibrational behavior of the "specimen-material" system without any loss at the contact interface 130'. Measurements with the test specimen 100, by comparison with the reference specimen, provide the pure effect of the interface 130'. For this, and as explained previously, it is necessary that the reference specimen 200 and the test specimen 100 have the same characteristics, namely an identical mass and stiffness distribution, an identical material, an identical geometry except for the attachment.

[0055] Furthermore, to ensure the accuracy of the deduction from the characterization of vibration damping at interface 130' (step 480), it is preferable that the measurements in steps 430 and 460 be carried out under the same conditions for the reference specimen 200 as for the test specimen 100. In particular, the thermal conditions must be identical for the reference specimen 200 and for the test specimen 100. To this end, the procedure 400 may include a preliminary temperature calibration step for both the reference specimen 200 and the test specimen 100 in a furnace. Indeed, standards relating to mechanical testing recommend equipping the specimens with a thermocouple probe, also called a thermocouple, whose role is to monitor the temperature of the specimen during the tests. However, the presence of a thermocouple generates a disruptive effect on the vibration mode.Since the invention aims to eliminate all external disturbances, method 400 proposes removing the thermocouple to eliminate its disruptive effects and compensating for the thermocouple's thermal effect by a temperature calibration step of the reference specimen 200 and the test specimen 100 in a furnace, for example at 550°C or 650°C. Specimens 200 and 100 are therefore heated according to the standard, but without the thermocouple. Temperature calibration in a furnace offers the advantage of a wide temperature range, reaching and even exceeding 800°C.

[0056] To implement the method 400, the test bench 300 is connected with a control and recording computing unit (not shown in the figures) which performs, in particular, the calculations for step 470, which compares the measurements taken with the reference specimen 200 and the test specimen 100, and for step 480, which determines the characterization of the vibration damping of the interface 130'. The computing unit can also determine, from the vibration damping value, the value of the coefficient of friction between the male and female parts in contact at the interface 130'.

[0057] In the preceding description, the test specimen 100, the characterization device, and its implementation method have been described for application to a turbine blade root and core assembly, the described test specimen being representative of this assembly. Those skilled in the art will understand, however, that the test specimen, the characterization device, and its implementation method can be applied to the characterization of the vibration damping of other turbomachinery parts in contact, such as, for example, gear parts, a blade damper, or any other assembly of parts in contact that adhere or slide with each other.

[0058] Thus, this process can be applied to a blade vibration damper.

[0059] This damper is a part positioned in contact with the small radius side of a blade face so that it is pressed against this face by the centrifugal field induced by the rotation of the blades around the motor axis. It can, for example, be positioned under the plate or under the heel. It dissipates energy by sliding and introduces a non-linearity of stiffness into the system, which results in a reduction of the vibration amplitude compared to operation without a damper.

[0060] A series of measurements can be carried out with a test specimen 100 in which a damper has been placed between the male part 130 and the female part 132.

Claims

Demands

1. A method (400) for characterizing the vibration damping of an adherent or sliding contact between two parts subjected to vibratory, oligocyclic and tribological stresses, comprising the following steps: a. installation (410) of a reference test specimen in a characterization device (300), b. adjustment (420) of the characterization device according to the measurements to be performed, c. measurements (430) of the vibratory behavior of the reference specimen, d. removal (440) of the reference test specimen, e. installation (450) of the test specimen in the characterization device (300), with the same adjustment as in step b), f. measurements (460) of the vibratory behavior of the test specimen, g. comparison (470) between the measurements taken in step c) for the reference specimen and the measurements taken in step f) for the test specimen, and h. determination (480), from the comparison of step g), of the characterization of the vibration damping of the contact zone.

2. A method according to claim 1, characterized in that the test specimen comprises: - a first head (111) and a first longitudinal bar (112), one end (112a) of the first longitudinal bar being fixed to the first head, the other end (112b) of the first longitudinal bar being free, - a second head (121) and a second longitudinal bar (122), one end (122a) of the second longitudinal bar being fixed to the second head, the other end (122b) of the second bar being free, and - a male-female type fastener (130) connecting the first longitudinal bar (112) with the second longitudinal bar (122) and comprising a male part (131) projecting from the free end of the first longitudinal bar and a female part (132) notched at the free end of the second longitudinal bar, the male part (131) and the female part (132) having complementary shapes, in adherent or sliding contact against the other.

3. Method according to the preceding claim, characterized in that the first longitudinal bar (112) has a shape substantially of a parallelepiped with rectangular cross-section, with a flare (114) near the first head (111) and a shoulder (113) at its free end.

4. A method according to any one of the preceding claims, characterized in that the second longitudinal bar (122) has a shape substantially of a parallelepiped with a rectangular cross-section, with a flare (124) near the second head and a buttress (123) at its free end.

5. Method according to claims 3 and 4, characterized in that the shoulder (113) of the first longitudinal bar and the buttress (123) of the second longitudinal bar extend in a plane perpendicular to a plane containing the flares (114, 124) of the first and second longitudinal bars.

6. A method according to any one of claims 1 to 5, characterized in that the male-female type attachment (130) substantially has a dovetail shape representative of a connection between a blade foot and a disk cavity of turbomachine, the male part (131) representing the blade foot and the female part (132) representing the disk cavity.

7. A method according to any one of the preceding claims, characterized in that it is implemented by a device (300) comprising a test bench (300) on which the test specimen (100) is mounted, said test bench comprising at least a vibrating pot (310) transmitting a vibratory excitation to the test specimen (100) and a static loading device (320) transmitting oligocyclic stresses to said test specimen.

8. Method according to claim 7, characterized in that the vibrating pot (310) is connected to the test specimen (100) by means of a threaded excitation rod (333'), in which a force measurement sensor (334) is integrated in an off-center manner.

9. Method according to claim 8, characterized in that the excitation rod (333') comprises a first and a second section (331, 332), joined together by an off-center linking system (335), the first section (331) and the second section (332) having different lengths.

10. A method according to any one of the preceding claims, characterized in that it comprises a preliminary step of temperature calibration of the reference specimen and the test specimen, in a furnace.

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